1. Field of the Invention
The present invention relates to a magnetostatic wave device, and more particularly, to a magnetostatic surface wave device used at a frequency of 1 GHz or less and comprising an oxide magnetic garnet single crystal.
2. Description of the Related Art
A magnetic garnet single crystal film has conventionally been used as a magnetic material for a bubble memory or an optical isolator. In addition, a magnetic garnet single crystal film has recently been applied to a microwave device, i.e., a magnetostatic wave device.
In the case of the magnetostatic wave device, a magnetic garnet single crystal film containing Fe is used, for example. Such a magnetic garnet single crystal film containing Fe is prepared by being epitaxially grown by a liquid phase epitaxial method so as to have a thickness of several tens .mu.m on a single crystal substrate expressed by the composition formula Gd.sub.3 Ga.sub.5 O.sub.12 (hereinafter abbreviated to a "GGG single crystal substrate"). The reason for using the GGG single crystal substrate is that it is most practical from the viewpoints of mass production and quality.
A magnetostatic wave is excited on a magnetic garnet single crystal film by applying a DC magnetic field to the magnetic garnet single crystal film and applying a high frequency magnetic field to the magnetic garnet single crystal film in a direction perpendicular to the magnetization direction. More specifically, when a DC magnetic field is applied perpendicularly or parallel with the surface of the magnetic garnet single crystal film, and a high frequency is applied to microstrip lines coupled with the magnetic garnet single crystal film, the precession of the magnetic moment occurs due to electron spins. A wave propagating through this precession is referred to as a magnetostatic wave.
The mode of the propagating magnetostatic wave is changed in accordance with the direction of the DC magnetic field applied. For example, when a DC magnetic field is applied in the direction parallel to the surface of the magnetic garnet single crystal film and perpendicular to the propagation direction of the magnetostatic surface, the internal magnetic field H.sub.i in the magnetic garnet single crystal film and the propagation frequency of the magnetostatic surface wave have the relationship expressed by the following equations (1) and (2)(Hewlett-Packard Journal Feb. 10-20, 1985): EQU f.sub.h =.gamma.(H.sub.i +2.pi.M.sub.s) (1) EQU f.sub.1 =.gamma.(H.sub.i .multidot.(H.sub.i +4.pi.M.sub.s)).sup.0.5 (2)
where H.sub.i =H.sub.ex -4.pi.Ma.sub.a +H.sub.a and f represents the frequency of the magnetostatic wave, .gamma. represents the gyro rotation ratio, 4.pi.M.sub.s represents the saturation magnetization, H.sub.ex represents the DC magnetic field applied from the outside, N represents the diamagnetic field coefficient, H.sub.a represents the anisotropic magnetic field, and f.sub.h and f.sub.1 are respectively the upper limit and the lower limits of the propagation frequency of the magnetostatic surface wave.
The above equations (1) and (2) indicate the important relations which define the frequency band of a magnetostatic surface wave device. By using these relations, the operation frequency and frequency band of a magnetostatic surface wave device, the DC magnetic field applied from the outside, the saturation magnetization of a material and operation temperature characteristics are determined.
In order to produce a magnetostatic surface wave device on the basis of the above relations, a magnetic garnet single crystal film having a diameter of, e.g., 2 to 3 inches is first epitaxially grown on a GGG substrate having the {111} crystallographic plane (hereinafter referred to as "the plane"). The obtained magnetic garnet single crystal film also has the {111} plane, and the {111} plane is perpendicular to the &lt;111&gt; crystal axis (referred to as "the axis" hereinafter) which is an easy magnetization axis of the magnetic garnet single crystal film. On the thus-prepared magnetic garnet single crystal film are formed microstrip lines having a line width of, e.g., 10 .mu.m by a method such as photolithography. The magnetic garnet single crystal film is then cut into chips. To each of the chips is mounted a magnetic circuit comprising a permanent magnet or the like. At this time, the magnetic circuit is formed so that a DC magnetic field is applied in parallel with the {111} plane and perpendicular to the &lt;111&gt; axis.
In the magnetic garnet single crystal film, the &lt;111&gt; axis has the lowest anisotropic energy and is in the state of the lowest energy level, and therefore, the easy magnetization axis of the magnetic garnet single crystal film is the &lt;111&gt; axis. On the other hand, the &lt;100&gt; axis is referred to as "the hard magnetization axis" and is in the state of the highest anisotropic energy level. These axes anisotropically affect the device characteristics.
However, neither the &lt;111&gt; axis nor the &lt;100&gt; axis exists on the &lt;111&gt; plane in the magnetic garnet single crystal film used in the magnetostatic wave device. Therefore, it has been thought that the magnetostatic wave device exhibits the same characteristics regardless of the direction in which a DC magnetic field is applied.
Experimental data has supported this idea. According to the results of ferromagnetic resonance measurements using a commercial electron spin resonance apparatus for measurement in the X-band (9.2 GHz) which permits measurement of magnetic anisotropy, it has confirmed that there is substantial no dependency of the horizontal resonance magnetic field on the crystal axes on the {111} plane with respect to a magnetic garnet single crystal film having the {111} plane.
Actually, it has also confirmed that, even if a DC magnetic field is applied in the direction of any low-index axis in the {111} plane, the device shows satisfactory characteristics at a frequency of 1 GHz and no difference was observed in the operation frequency and the band of the magnetostatic surface wave device regardless.
However, when the magnetostatic surface wave device is used at the frequency lower than 1 GHz, there arise some problems. Specifically, in order to lower the operation frequency of the magnetostatic surface wave device, it is necessary to reduce the applied DC magnetic field or to decrease the saturation magnetization of the material in accordance with the relations expressed by the above equations (1) and (2). Nonetheless, particularly at a frequency of less than 1 GHz, the strength of the applied DC magnetic field significantly influences the stability of magnetization. Therefore, when the applied DC magnetic field is weakened so as to set the central frequency at 1 GHz or less, specifically about 500 MHz, the magnetic garnet single crystal film is not sufficiently magnetized, which makes the characteristics of the magnetostatic surface wave device unstable and results in that a magnetostatic surface wave device for practical use cannot be obtained.
For the aforementioned reasons, there arises a demand for a magnetostatic wave device which can exhibit stable characteristics at a frequency of 1 GHz or less.